Laser processing machines direct a laser beam at a work piece to cut, drill, weld, or otherwise machine the work piece. Many laser processing machines use a pair of galvanometers to rotate mirrors to direct the laser beam along two perpendicular axes. Such systems incorporate a scan lens, also known as an F-theta lens, which linearizes the relationship between mirror angle and laser beam position, produces a flat field, and focuses the laser beam to a spot on the work piece. Collectively, the galvanometers, mirrors, and the F-theta lens are often referred to as a scan head.
Laser processing machines that incorporate the scan head can operate at relatively high speeds. However, the size of the work area is limited by the size of the F-theta lens and the rotational limits of the galvanometers. To enlarge the working area beyond the limits of the scan head, some laser processing machines include a second set of actuators that move either the scan head or the work piece such that the working area of the scan head can be made to overlap different parts of the overall processing area of the machine.
Position sensitive detectors (PSDs) are photodiodes that are used as optical position sensors. PSDs include two classes. Segmented PSDs are common substrate photodiodes that are partitioned into either two or four active regions, for one or two dimensional measurements respectively, separated by inactive gaps. Each region provides a photocurrent output. Lateral effect PSDs are single element photodiodes with no inactive regions that provide two current outputs for each sensing axis. For both segmented and lateral effect PSDs, the position of the beam on the surface of the detector is determined by comparing the photocurrents flowing to or from each of the outputs of the PSDs.
Typically, a feedback control system is used to control the motion of the various actuators in the laser processing machine. Feedback devices, such as encoders and resolvers, provide position and angle information about the axes to the controller. The ideal relationship between the actuator positions and the position of the laser beam on the work piece is known a priori, thereby allowing the controller to generate positioning command signals for the actuators that result in proper positioning of the laser beam. During operation, the controller determines the commands given to the actuators in order reduce a difference between the desired actuator position and the current actuator position, commonly known as the error, to zero.
It is important to note that the feedback control system does not directly control the position of the laser beam on the work piece. Rather, the feedback system controls the positions and angles of various actuators that direct the laser beam to the desired position. This distinction is a major source of error in the position of the laser beam. The feedback control system can only eliminate errors for which feedback is received. Because there is no feedback on the actual position of the laser beam, discrepancies between the ideal and actual relationship of actuator position to laser beam position cannot be eliminated by the feedback action of the control system. Errors of this sort can only be addressed by calibration.
Most laser processing machines require calibration to achieve required levels of accuracy. In particular, laser processing machines that incorporate a scan head require calibration to compensate for many factors including errors in beam alignment, errors in mirror alignment, errors in the mirror angle feedback devices, F-theta lens alignment, distortion in the F-theta lens, and other sources.
Calibration systems and methods for these error sources generally have two primary characteristics. First, the systems include a measurement apparatus that records the actual position of the laser beam for one or more desired positions during calibration. Second, they include some method for processing the resulting stored data, and then using the data to correct the actuator commands during on-line operation of the machine, such that the accuracy of the position of the laser beam on the work piece is improved. Numerous calibration techniques exist in the art, as illustrated by the following examples.
U.S. Pat. No. 4,918,284-1990 describes a laser trimming device. The device uses a camera to acquire images of the work surface via the deflection unit and the F-Theta lens, in conjunction with a specially patterned calibration plate. The deflection unit directs the beam at the calibration plate. Images of the plate are analyzed to determine a difference between desired and actual positions of the laser beam.
U.S. Pat. No. 6,808,117 describes a method and apparatus for calibrating a laser chip scale marking machine. The apparatus uses a camera for determining positioning errors. However, that camera does not image via an optical path of the laser beam. Rather, the camera is located on a side of the work piece opposite the F-Theta lens. The beam is directed at a target installed in place of the work piece and the position where the beam impinges on the target is observed with the camera. The error is measured as the difference between the desired and actual position of the laser beam on the target.
U.S. Pat. No. 7,006,237 describes a method for calibrating a laser processing machine. Holes are marked on a test work piece and the positions of the holes are measured using a camera during off-line operation. The difference between the desired and actual positions is used by a controller to determine an unknown parameter matrix. This matrix is used during on-line operation to determine the optimal command value to direct the laser beam to the desired position on the work piece.
U.S. application Ser. No. 13/346,809 filed by assignee of the present application describes calibrating a laser cutting machine using a calibration plate arranged to simulate the work piece. Two quad photodiodes are positioned to receive light from the surface of the calibration plate via the optical path of the laser beam, and convert an image of a portion of a surface of the calibration plate into electrical signals for calibration purposes.